Steels_ Metallurgy and Applications, Third Edition

Low-carbon structural steels 173

stiffeners in the form of rolled sections (bulb or plain flats) are used to reinforce
these areas and also the hull sides. Plating is also used in the longitudinal and
transverse partitioning walls or bulkheads, which also contribute to the strength
of a ship with respect to buckling.
For the most part, a ship's hull is constructed from Grade A steel but Lloyd's
Rules distinguish between the material grade requirements for different parts
of the hull. As shown in Table 2.12(a), five classes of material are identified
in ascending order of fracture toughness which are translated into steel grades,
according to thickness requirements, in Table 2.12(b). A schematic illustration
of the location of Grades A, D and E in the midship section of a large tanker is
shown in Figure 2.18. Thus the use of the tougher Grades D and E is confined
mainly to the more highly stressed deck and bottom regions whereas Grade A is
adequate for the sides of a vessel.
The use of higher-strength steels in shipbuilding is attractive from two aspects:

1. Lower construction costs- from reduced steel weight and lower fabrication
   costs.


2. Lower operating costs- from reduced weight/lower fuel costs or higher
   carrying capacity for the same constructed weight.

Given the depressed state of the shipbuilding market, the former is probably the
more important but the reduction in thickness that can be tolerated is governed
by modulus and deflection considerations. From the analogy with a simple beam,
the deflection of a ship is a function of the ratio of length to depth (LID) and
also the thicknesses of the plates used in the girder construction. Caution has
therefore had to be exercised in the use of higher-strength steels, in reduced
sections, in order to limit the deflection to within reasonable bounds. However,
as experience has been gained, further advantage has been taken of the higher-
strength grades. Currently, Lloyd's rules provide KL factors for the determination
of the hull girder section modulus in higher-strength steels and the values are
shown in Table 2.13. For mild steel (235 N/mm 2 min. YS), the KL factor = 1.0
and therefore significant reductions in the section modulus are now permitted

E\~--' D I

A

W

E

1

A

~ D 1~

9 E

Figure 2.18 Midships section showing distribution of Grades A, D and E